The present invention relates to a technique employable effectively for multistage high frequency power amplifier circuit devices, each having a plurality of semiconductor amplification elements connected in a cascade, as well as for such radio communication apparatuses as portable telephones, etc. in which such a high frequency power amplifier circuit device is built respectively. More particularly, the present invention relates to a technique for improving the controllability of an output power (gain) with use of a power control signal voltage of the high frequency power amplifier circuit device and the efficiency of the device at a low power output.
In such radio communication apparatuses (mobile communication apparatuses) as mobile telephones, portable telephones, etc., a multistage high frequency power amplifier circuit device is built in the transmission side output stage respectively. The high frequency power amplifier circuit device includes semiconductor amplification elements such as MOSFETs (Metal Oxide Semiconductor Field-Effect-Transistors) and FaAs-MESFETs, etc., which are connected in a cascade. In the high frequency power amplifier circuit device, the semiconductor amplification element in the last stage is usually composed of discrete parts (an output power MOSFET, etc.) and the semiconductor amplification element in the preceding stage and the bias circuit are often integrated into a semiconductor integrated circuit formed on one semiconductor chip. Hereinafter, a component in which a semiconductor integrated circuit that includes semiconductor amplification element parts, the bias circuits, capacity elements, etc. are integrated will be referred to as a high frequency power amplifier module or simply as a module.
And, a portable telephone system is generally configured so as to change the output (transmission power) according to the ambient conditions with use of a power level command signal received from a base station so as not to cause radio interference in the communications with another portable telephone. For example, in the case of the cellular portable telephone such as the 900 MHz band standard method employed in the United States of America, the GSM (Global System for Mobile Communication) method employed in Europe, etc., the high frequency power amplifier module in the transmission side output stage is configured so that the gate bias voltage of each output power element is controlled so as to output a power required for talking with use of the output voltage Vapc of the APC (Automatic Power Control) circuit.
In addition, for a portable telephone, employment of a high efficiency high frequency power amplifier module is a very important factor for deciding a talking time and a waiting time, that is, an operating life of the battery. This is why the performance of the mutual conductance, etc. among the semiconductor amplification elements of a module has been improved to achieve such the high efficiency.
Japanese Patent Application No. Hei 11(1999)-275465 discloses a radio communication apparatus in which a multistage high frequency power amplifier module is built. The module includes a plurality of MOSFETs that are connected in a cascade. This radio communication apparatus improves the controllability of the output power Pout (to suppress an increase of the Pout with respect to an increase of the Vapc, that is, xcex94Pout/xcex94Vapc) with use of bias means provided to generate a gate bias voltage Vg so as to minimize the variation of the output power Pout with respect to the Vapc around the threshold voltage Vth of the MOSFET in each amplification stage according to the power control signal voltage Vapc generated on the basis of its body power control signal.
FIG. 23 shows a bias circuit that controls the gate bias voltage in each stage in the three-stage high frequency power amplifier module disclosed in the above official gazette non-linearly with respect to the power control signal voltage Vapc. As shown in FIG. 23, the bias means (bias circuit) of the three-stage high frequency power amplifier module is composed of a plurality of resistors R01 to R04 connected serially and a transistor Q01 connected to those resistors R01 to R04 through a diode respectively. The resistor""s voltage dividing ratio is set so as to obtain the maximum power at a position where the Vapc becomes as large as about 2 V, thereby the gate bias in each stage is decided. The diode-connected transistor is usually transformed into an MMIC (Microwave Monolithic IC) with use of the same semiconductor process technology as that of the amplification MOSFET.
FIGS. 24 and 25 are graphs for denoting a relationship between the power control signal voltage Vapc in the bias means shown in FIG. 23 and the gate bias voltage Vg in each stage and a relationship between the power control signal voltage Vapc obtained by the present inventor through a test of a module composed as shown in FIG. 23 and the output voltage Pout. The amplification MOSFETs used in the above test have a threshold voltage Vth of about 0.8 V respectively.
In the bias circuit disclosed in the above gazette, the gate bias voltage Vg in each stage is varied linearly (Vg=Vapc), since the control voltage Vapc is output as Vg1 to Vg3 with no change. This is because the Q01 is off until the control voltage Vapc reaches the threshold voltage of the diode-connected transistor Q01 as to be understood from the graph shown in FIG. 24 that shows a relationship between the control voltage Vapc of this circuit and the gate bias voltage Vg in each stage. The gate bias voltage Vg is then varied non-linearly.
In the module shown in FIG. 23, however, the threshold voltage Vth of the amplification MOSFET becomes almost the same as the threshold voltage of the diode-connected transistor formed in the same process, so that the amplification MOSFETs in all the stages are driven almost concurrently, thereby the output power Pout changes suddenly. In other words, as to be understood from the graph shown in FIG. 25, the output power curve that denotes the characteristics of the output power is inclined sharply around 0 dBm and the output power Pout changes significantly due to a slight change of the voltage Vapc. This is why the controllability of the output power has not been improved to the required level. In addition, the output power Pout also comes to change more sharply depending on a variation of the threshold voltage Vth of the amplification MOSFETs.
In prior to the present invention, the inventor has examined a bias circuit composed of a plurality of resistors R01 to R04 connected serially as shown in FIG. 20. The bias circuit is used to linearly control the gate bias voltage in each stage in the three-stage high frequency power amplifier module with respect to the power control signal voltage Vapc. FIGS. 21 and 22 are graphs for denoting a relationship between the power control signal voltage Vapc and the gate bias voltage Vg in each stage in the bias circuit shown in FIG. 20 and a relationship between the power control signal voltage Vapc obtained by the inventor through a test of the module composed as shown in FIG. 20 and the output power Pout. The threshold voltage Vth of the amplification MOSFETs used in the above test is about 0.8 V.
As shown in FIG. 20, a resistance ladder circuit is generally used as a bias circuit for linearly controlling an object voltage. The voltage dividing ratio of the resistor decides an inclination of the gate bias voltage Vg. As to be understood from the graph shown in FIG. 22, the curve that denotes the characteristics of the output power inclines sharply around an output power of 0 dBm. The output power Pout changes significantly at a slight change of the voltage Vapc even when the bias circuit as shown in FIG. 20 is used. This means that the output controllability is not so good.
Under such circumstances, it is an object of the present invention to provide a high frequency power amplifier circuit device that can obtain excellent controllability of an output power with use of a power control signal voltage.
It is another object of the present invention to provide a high frequency power amplifier circuit device that can obtain excellent controllability of an output power with use of a power control signal voltage, as well as a high efficiency at a low output power.
It is still another object of the present invention to provide a high frequency power amplifier circuit device that enables radio communication apparatuses to obtain a longer talking time and a longer operating time of its battery respectively.
These and other objects, features of novelties of the present invention will become more apparent by referring to the following description and appended drawings.
Next, the typical items of the present invention disclosed in this specification will be described briefly.
In a high frequency power amplifier circuit device provided with multiple output stages in which a plurality of first semiconductor amplification elements (Q1 to Q3) are connected in a cascade and a bias control circuit (10) that drives each of the plurality of first semiconductor amplification elements according to a control voltage, the bias control circuit is configured so as to apply a predetermined initial bias voltage to the control terminal of each of the plurality of first semiconductor amplification elements, thereby supplying a current to each of the semiconductor amplification elements even when an input control voltage is practically xe2x80x9c0xe2x80x9d and the initial bias voltage applied to the plurality of first semiconductor amplifier elements is controlled so as to become higher gradually from the first stage to the last stage.
According to the above described means, for example, in a radio communication apparatus, the sharp change of the output power caused by the control voltage is eased, thereby the controllability of the output power is improved, since the change rate of the bias voltage in each of the first semiconductor amplifier elements can be reduced in an area where the control voltage is low, especially in an area around the threshold voltage where the gain change in each of the first semiconductor amplification elements is significant when the bias of the first semiconductor amplification element in each stage is controlled according to a control voltage (power control signal voltage Vapc) output from an automatic power control circuit (APC circuit) via a bias control circuit according to a power level command signal.
Furthermore, the above described means enables the bias conditions (bias starting point and bias voltage change rate) of the first semiconductor amplification element in each stage to be set at a desired valance and the first semiconductor amplification element in the last stage to be driven very efficiently, so that the operating current of the high frequency power amplifier circuit device is reduced and the talking time and the working life of the battery in the subject portable telephone are extended.
The change rate of the bias voltage to be applied to the control terminal of each of the plurality of output semiconductor amplification elements should preferably be set lower gradually from the first stage to the last stage while the first voltage is higher than the threshold voltage of the semiconductor amplification elements and higher gradually from the first stage to the last stage when the first voltage is exceeded. Consequently, it is possible to improve the efficiency of the high frequency power amplifier circuit device of the present invention when the control voltage is low and drive the circuit device so as to obtain a high output power when the control voltage is high. The first voltage mentioned above should preferably be 0.1 to 0.5 V higher than the threshold voltage of the first semiconductor amplification elements.
The change rate of the bias voltage applied to the control terminal of each of the plurality of first semiconductor amplification elements is controlled so as to become higher when the control voltage is higher than the second voltage, which is higher than the first voltage. Consequently, the circuit device can be driven so as to obtain a desired output power more efficiently.
The bias control circuit controls the bias voltage applied to each of the first semiconductor amplification elements to xe2x80x9c0xe2x80x9d practically until an input control voltage reaches a third voltage, which is lower than the first voltage, then applies a predetermined initial bias voltage to each of the first semiconductor amplification elements when the control voltage reaches the third voltage. Consequently, it is possible to turn off the high frequency power amplifier circuit device to minimize the output power (leak power, isolation) when the control voltage is almost xe2x80x9c0xe2x80x9d. In addition, it is possible to generate a dead band that can reduce the current (leak current) that flows in the high frequency power amplification circuit device when the control voltage is 0 V.
The bias control circuit includes a current buffer circuit being composed of a voltage-current conversion circuit (11) that converts the control voltage to a current; a first resistor (R12) that converts the current supplied from the voltage-current conversion circuit to a voltage; a control voltage generation circuit (12) that includes a first constant current source (Ic) and a second semiconductor amplification element (Q12) connected serially to the first constant current source and enabled to generate a voltage equivalent to a threshold voltage of the second semiconductor amplification elements; a third semiconductor amplification element (Q16) that generates a current according to a synthesized voltage of the voltage generated by the control voltage generation circuit and the voltage converted by the first resistor; and a second constant current source (Ir) connected to the control terminal of the third semiconductor amplification element and enabled to pull in a current supplied from the voltage-current conversion circuit; a current buffer circuit (13) that supplies a current having the same characteristics as those of the current flowing in the third semiconductor amplification element; current-voltage conversion means (R13) that converts a current flowing in the current buffer circuit to a voltage to drive the first semiconductor amplification elements. The current of the second constant current source is varied among the first semiconductor amplification elements, thereby the control voltage level on which a current begins flowing in each of the first semiconductor amplification elements is varied among the first semiconductor amplification elements. Consequently, because the bias voltage change rate of each of the first semiconductor amplification elements can be reduced in an area where the control voltage is low, especially in an area around the threshold voltage where the gain change rate of the semiconductor amplification elements is large, the controllability of the output power can be improved. In addition, the control voltage level on which a current begins flowing in each of the first semiconductor amplification elements can be varied only by changing the current value of the second constant current source (Ir). It is thus possible to obtain output characteristics in accordance with the target first semiconductor element.
Furthermore, the high frequency power amplifier circuit device includes a plurality of semiconductor amplifier elements (Q10, Q20, and Q30), each being connected to one of the plurality of first semiconductor amplification elements so as to form a current mirror circuit. The above described bias control circuit is composed of a voltage-current conversion circuit that converts the control voltage to a current; a first resistor that converts a current supplied from the voltage-current conversion circuit to a voltage; a control voltage generation circuit provided with a first constant current source and a second semiconductor amplification element connected serially to the first constant current source and enabled to generate a voltage equivalent to the threshold voltage of the second semiconductor amplification element; a third semiconductor amplification element that generates a current according to a synthesized voltage of the voltage generated by the control voltage generation circuit and the voltage converted by the first resistor, and a second constant current source connected to the control terminal of the third semiconductor amplification element and enabled to pull in a current supplied from the voltage-current conversion circuit. The bias control circuit supplies a current to each of the semiconductor amplification elements connected to one of the first semiconductor amplification elements to form a current mirror respectively to drive each of the first semiconductor amplification elements. The current has the same characteristics as those of the current flowing in the third semiconductor amplification element. And, the current supplied from the second constant current source is set so as to be different among the plurality of first semiconductor amplification elements, thereby a control voltage level on which the current begins flowing in each of the first semiconductor amplification elements comes to differ from others.
Consequently, it is possible to vary the control voltage level on which the current begins flowing in each of the first semiconductor amplification elements only by changing the current value of the second constant current source (Ir), thereby it is possible to obtain output characteristics easily in accordance with the target semiconductor element. In addition, because each of the first semiconductor amplification elements is driven by a current provided with predetermined characteristics, the high frequency power amplifier circuit device of the present invention can have output characteristics that are free of the variation of the characteristics of the threshold voltage, etc. of the first semiconductor amplifier elements.
The above described bias control circuit includes a plurality of first current sources (Q42, Q43, and Q44) that supply a current in proportion to the control voltage and a plurality of second current sources (Q46, Q49, and Q52), each supplying a current different from others regardless of the value of the control voltage. The bias control circuit synthesizes currents obtained by subtracting the current of the corresponding second current source from each of the plurality of first current sources to generate a control current (Ia1) and drive each of the first semiconductor amplification elements with a voltage converted from the control current or with a current whose characteristics are the same as those of the control current, thereby the change rate of the bias voltage is changed according to the control voltage. The bias control circuit can thus vary the control voltage level on which a current begins flowing in each of the semiconductor amplification elements even in such a configuration.
The bias control circuit includes a plurality of differential amplifier circuits (GM-AMP1 to GM-AMP4), each receiving a common control voltage via one input terminal thereof and first and second voltages via another input terminal as compare voltages, as well as a plurality of current circuits (Q31 to Q38), each supplying a current according to the output of each of the plurality of differential amplifier circuits. The plurality of first semiconductor amplification elements are driven with a voltage converted from a current generated by synthesizing the currents supplied from the plurality of current circuits or driven with a current having practically the same characteristics as those of the synthesized current, thereby the bias change rate is changed in accordance with the control voltage. Even in such a configuration, the bias control circuit of the present invention can vary the control voltage level on which the current begins flowing in each of the semiconductor amplification elements among the semiconductor amplification elements.
The voltage-current conversion circuit includes a differential amplification circuit (114) that receives the control voltage (Vapc) via one input terminal thereof and a comparison circuit (113) that detects whether or not the control voltage has reached the predetermined voltage and a switch element (Q26) is provided in parallel to a load element of the differential amplification circuit and the switch element is turned on/off by an output of the comparison circuit, thereby no current is flown to any of the first semiconductor amplification elements until the control voltage reaches a predetermined voltage and the predetermined initial bias voltage is applied to the first semiconductor amplification elements so as to flow a current in each of them after the control voltage reaches the predetermined voltage. Consequently, when the control voltage is almost xe2x80x9c0xe2x80x9d, the high frequency power amplifier circuit device can be turned off to minimize the output power. In addition, it is possible to easily realize a circuit that generates a dead band that reduces the leak current flowing in the high frequency power amplifier circuit device when the control voltage is 0 V.